1. Technical Field of the Invention
The present invention relates generally to the field of delay line frequency discriminators. More particularly, the invention relates to an electro-optic delay line frequency discriminator, and to a method for reducing noise in an electro-optic delay line frequency discriminator.
2. Background of the Invention
A frequency discriminator (hereinafter referred to as a “discriminator”) performs frequency demodulation, producing an output signal that is proportional to an input frequency. A particular type of discriminator, a delay line discriminator, uses a transmission line to produce a delay. The basic principle of operation of a delay line discriminator is that the phase shift of the output signal of a delay line is proportional to frequency. The phase of the delayed signal is compared to the phase of the undelayed signal at the input to the delay line by a phase detector. The output of the phase detector is proportional to the phase difference between its two outputs; and, hence, is also proportional to the input frequency, as required for proper discriminator action. Delay lines are used in many applications to delay one signal with respect to another. The lowest modulation frequency that can be demodulated by a discriminator is determined by the length of the delay.
Electrical delay line discriminators implement the delay with an electrical transmission line; however, if good low frequency response is needed, the electrical transmission line is generally quite long and introduces substantial signal loss, resulting in poor signal-to-noise ratio. Optical fibers are known that introduce very little signal loss; and, as a result, electro-optic delay line discriminators have been developed that have an electrical input and output, but that have internal electrical-to-optical and optical-to-electrical conversions to allow the delay to be implemented optically with an optical fiber transmission line.
An electro-optic delay line discriminator usually employs a laser to provide an optical signal source; however, the output of a laser exhibits relative intensity noise (RIN). In conventional electro-optic delay line discriminators, this laser noise degrades the signal-to-noise ratio at the output of the discriminator. There is a trade-off in laser design between low RIN versus size, power complexity and cost.
There is, accordingly, a need for an electro-optic delay line discriminator that is substantially immune to laser RIN.